Our objective is to understand the major stage specific antigens of Toxoplasma gondii and their role in immunity. The three stages to be investigated are the tachyzoite (asexual invasive stage), bradyzoite (chronic encysted stage) and the oocyst/sporozoite (feline excreted postgametogenesis stage). The first specific aim is to determine which antigens of the tachyzoite elicit a humoral immune response. Parasiticidal monoclonal antibodies generated against tachyzoite antigens will be used to purify antigens critical to protective humoral immunity and to select monoclonal antibody resistant mutants. Purified antigens will be used to screen for congenital toxoplasmosis. Minor antigenic differences among different tachyzoite strains will be identified and their importance in strain specific immunity will be assessed. We will investigate by molecular cloning the surface antigens of T. gondii that elicit a host parasiticidal response. These studies will explore the potential of synthetic peptides representing these antigens in eliciting immunity to toxoplasmosis. In the absence of adequate cell mediated immunity toxoplasmosis is a serious infection of immunosuppressed individuals, especially victims of AIDS. The tachyzoite antigens that elicit the cellular response may differ from those that stimulate humoral immunity. Specific antigens that induce in vitro T-cell proliferation will be identified and the response frequency determined by limit dilution analysis. Proliferating lymphocytes will be assayed for cytotoxic ability. Supernatant from antigen primed T-cell cultures will be assayed for parasite growth inhibitory factors such as gamma interferon or possibly antigen specific lymphokines. The third aim is to examine the stage specific antigens of the bradyzoite. These antigens will be identified and characterized with monoclonal antibodies. Once characterized these antigens may be useful in evaluating the duration of infection in high risk individuals such as pregnant women. Parasite transformation from tachyzoite to bradyzoite will be studied in vitro using a monoclonal antibody model. The fourth specific aim is to determine if stage specific antigens elicit an IgA immune response. IgA antibody in toxoplasmosis has only been studied in a cursory manner. The in vivo IgA antibody response in serum and CSF to parasite antigens will be determined. The secretory IgA response in cats to stage specific parasite antigens will be evaluated and the ability of purified antigens to elicit a protective response assessed.